Current-mode control of DC to DC converters has many advantages over traditional pulse width modulated control including providing better dynamic performance and offering the ability to operate multiple converters in parallel. The main disadvantage associated with this method is the so called "open loop instability" which appears if the converter is operated at a duty cycle greater than 50%. Hysteretic current-mode control also overcomes open loop instability. However, the frequency changes as the input voltage of the converter changes. The turn on and turn off of the converter power stages creates noise spikes which may cause problems when running two or more converters in parallel.
Hysteretic current-mode control is one of the ways to avoid open loop instability, maintain a stable operating frequency and still retain all the advantages of the current-mode control.
It is an objective of the present invention to provide hysteretic current-mode control DC to DC converters with stable operating frequency.
It is another object of the present invention to provide a nonlinear circuit for hysteretic current-mode control DC to DC converters which establish appropriate hysteretic current limits.
It is still another object of the present invention to provide duty cycle proportional control for hysteretic current-mode control DC to DC converters.
It is yet another object of the present invention to provide a circuit to sense the boundary between heavy and light mode operation in hysteretic current-mode controlled DC to DC converters.
It is a further object of the present invention to provide a circuit which implements hysteretic average current control in heavy mode and constant frequency peak current control in light mode for current-mode DC to DC converters to achieve constant frequency operation in both heavy and light operating modes.